Here is my way of doing it. It works best. I've been surprised by all the trouble people were having when doing it another way..
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There's another solution too if you don't want to use litecom: open up the icon file in Preview, when it opens, press Command + C on your keyboard. Quit preview. Select the disk who's icon you want to change and Command + I, click the original icon (top left) in the window that opens, a blue glow effect should then appear. Press Command+V on your keyboard. Your icon is changed.